To measure a laser frequency, a method of using an optical frequency comb device is proposed in recent years. For example, as described in Japanese Patent Application Laid-Open No. 2007-256365 (hereinafter called Patent Literature 1), the use of the optical frequency comb device facilitates measurement of the oscillation frequency of a laser with high accuracy. This optical frequency comb device is a device that outputs a laser having a comb-shaped spectrum with a repetition frequency (longitudinal mode spacing) of frep, and has the property of having the precisely equal frep in any frequency band. (See, for example, H. Inaba, Y. Nakajima, F. L. Hong, K. Minoshima, J. Ishikawa, A. Onae, H. Matsumoto, M. Wouters, B. Warrington, and N. Brown, “Frequency Measurement Capability of a Fiber-Based Frequency Comb at 633 nm,” IEEE Transactions on Instrumentation and Measurement, vol. 58, pp. 1234-1240, April 2009, which is hereinafter called Non-Patent Literature 1.)
FIG. 1 shows the frequency spectra of an optical frequency comb (also called optical comb) and a laser to be measured.
An oscillation frequency νn in the n-th comb mode of the optical comb can be represented by the following formula:νn=n·frep+fCEO  (1).
In the formula, fCEO represents a carrier envelope offset frequency (hereinafter called CEO frequency), and “n” represents a mode order that indicates the number of an order of a mode with setting an initial mode as the 0-th mode.
Here, by interference between the laser to be measured (having a frequency of νlaser) and the optical comb, a frequency difference fB therebetween is observed as a beat signal, as represented by the following formula (2):fB=νlaser−νn  (2).
Thus, the frequency νlaser can be obtained by the following formula (3) using the formulas (1) and (2):νlaser=n·frep+fCEO+fB  (3).
Therefore, if the repetition frequency frep and the CEO frequency fCEO of the optical frequency comb are synchronized with a standard frequency (for example, a frequency synchronized with coordinated universal time) and the beam frequency fB is measured, it is possible to accurately measure (calculate) the absolute frequency νlaser of the laser to be measured by setting the appropriate integer “n”.
FIG. 2 is a schematic view of a spectrum of a beat frequency signal that occurs when a photodetector receives the interference between the optical frequency comb and the laser to be measured. The beat frequency signal has peaks at the repetition frequency frep by the interference among the modes of the optical frequency comb itself and a frequency fB′ (=frep−fB) conjugate to the beat frequency fB, in addition to a peak at the beat frequency fB to be measured. Moreover, relative to fundamentals of these three frequencies, a harmonic occurs repeatedly at every frequency of frep. Therefore, in order to measure the beat frequency fB with high accuracy, it is required to cut off unnecessary frequency components other than the beat frequency fB by using a band pass filter (BPF) for an RF frequency signal.